1. Field of the Invention
The present invention relates to a miniature drill chuck comprising a drill chuck housing comprised of a drill chuck head and a drill chuck plug arranged at a drive side and having a central thread for a machine spindle, wherein the drill chuck head has an inner wall conically converging in the direction to the receiving opening for a drilling tool, a milling tool, or a grinding tool, on which matching conical clamping jaws are guided longitudinally slidably and under a pretension force acting in the circumferential direction, wherein the clamping jaws are loaded on their rearward surface by a thrust member which is moved in the direction toward the receiving opening by threading the drill chuck housing into the machine spindle with simultaneous entrainment of the clamping jaws moving toward one another into the clamping position.
2. Description of the Related Art
Such miniature drill chucks are known in general (Hitachi or Goodell).
The principle of such miniature drill chucks resides in a thread which is provided at the backside in the drill chuck housing and into which a corresponding thread of the machine spindle is threaded.
By a slow advancing motion of the threaded pair a movement in the direction to the receiving opening is exerted onto the clamping jaws arranged within the drill chuck head which allows the clamping jaws to move toward one another into their clamping position.
This is realized by a movement of the clamping jaws against a pretension force acting in the circumferential direction so that upon opposite rotational movement of the miniature drill chuck relative to the machine spindle the clamping jaws are returned from their clamping position into the release position.
Such miniature drill chucks must be differentiated from conventional drill chucks where the advancing movement of the clamping jaws is initiated by means of a drill chuck ring with gearwheel in connection with a special drill chuck key.
Since such miniature drill chucks are designed particularly for high to very high rotary speeds, the subject of the present invention is primarily used in miniature drill chucks with a clamping width up to approximately 6 mm diameter and for rotary speeds of above 10,000 rpm.
The special problem in regard to these high rotary speeds is the vibration tendency of the entire drive train from the motor to the tool which also contributes to a vibration excitation of the entire system, in particular, when passing through the critical rotary speeds. It is therefore particularly important to manufacture such miniature drill chucks with low characteristic vibration over the entire rotary speed range.
In the context of the present invention, very high rotary speeds are to be understood as rotary speeds above 20,000.
Under these conditions, it is an object of the invention to improve the entire known miniature drill chuck such that it produces an excellent clamping action even at rotary speeds above 20,000 rpm with reduced vibration over the entire rotary speed range.
This object is solved by the invention in that the thrust member is longitudinally moveable in a guide bore of the drill chuck housing and is circumferentially centered along its entire stroke.
Important for the invention is a tight fit between the outer diameter of the thrust member and the correlated inner bore of the drill chuck housing which reliably prevents a radial movement of the thrust member in the drill chuck housing in all operational states.
For this purpose, a guide bore is provided in the drill chuck housing within which the thrust member is circumferentially guided and remains guided about its entire available stroke.
Since the thrust member must be supported longitudinally slidably during the advancing movement of the clamping jaws within the drill chuck housing, the requirement in regard to a tight guiding fit between thrust member and guide bore must be realized over the entire stroke of the thrust member.
This results in the advantage that the dynamic balancing conditions of the miniature drill chuck are maintained over the entire rpm range to be used up to the maximum rpm. The special feature of the invention thus resides in a fit of the thrust member within the guide bore which is radially tight but axially moveable.
This requirement can be fulfilled in regard to the miniature drill chucks in that, as a result of only minimal clamping diameters of such miniature drill chucks, radial movements of the thrust member within the guide bore have practically no effect up to highest rotary speeds.
Expediently, the guide bore is provided in the drill chuck plug. This measure has the advantage of a very light drill chuck head so that the masses positioned with relatively great spacing to the machine spindle remain relatively minimal.
When on the drill chuck plug a circumferential knurling is provided by which a press fit can be a achieved in the correlated bore of the drill chuck head, the mounting expenditure for the drill chuck housing can be significantly reduced. In spite of this, the circumferential knurling provides always a precise centering of the drill chuck plug within the drill chuck head.
Important in connection with the invention is a precise guiding of the thrust member in the drill chuck housing over its entire stroke. For this purpose, it is suggested that the length of the thrust member in the guide bore is at least as great as the diameter of the thrust member. This measure suppresses the tendency of too-short thrust members against canting and ensures an unbalance-free operation.
In addition, it may be provided that the thrust member is weight-reduced with rotational symmetry in order to additionally reduce the moved masses. For this purpose, the thrust member can have an outer groove on its circumference. Optionally, the thrust member can also be of a hollow sleeve shape. As a supplement or in addition, it can also be provided that the drill chuck plug is furnished with corresponding measures for reducing its weight. This can easily include circumferential grooves, radial bores, material cutouts, as long as they are arranged with rotational symmetry.
These measures for reducing the rotating mass concern therefore primarily the thrust member. However, corresponding measures for reducing weight can be provided also at least on the drill chuck plug or the drill chuck head, which measures are to be included in the present invention.
In order to achieve a further improvement of the guiding action of the thrust member, the end of the thrust member facing the spindle can be machined to a reduced outer diameter relative to the inner diameter of the drill chuck plug so that between the drill chuck plug and the thrust member an annular gap space is produced. A hollow-cylindrical machine spindle can be screwed into this annular gap space which is supported with an outer thread on a corresponding inner thread of the drill chuck plug while its end area is used at the same time for moving the thrust member.
For centering the thrust member, additionally a central truncated cone surface can be provided which is arranged on the bottom of the annular gap space. A matching truncated cone surface of the drive spindle is to be seated on this central truncated cone surface.
In order to make available an additional lever arm for obtaining the clamping position of the clamping jaws, key surfaces which are positioned opposite one another with regard to rotational symmetry can be provided on the drill chuck head or the drill chuck plug.